Generally, it is strongly desired that an electrolytic capacitor which is one of the types of capacitors used for the secondary side circuit part of a power source circuit and circuit part around the CPU of a personal computer has a small-size and a high capacity. Further, it is demanded of this type of electrolytic capacitor to be improved in a low ESR (equivalent series resistance) in correspondence to high frequency. In order to attain the low ESR, a solid electrolytic capacitor using, as the solid electrolyte, a functional polymer having a high electroconductivity has been developed and put to practical use. Also, there is an increased demand for a polymer solid electrolytic capacitor.
The conventional solid electrolytic capacitor is so constituted that the surface area of an electrode can be enlarged to attain a high capacity. As the structure of the solid electrolytic capacitor to attain this, a cylinder type obtained by winding a continuous electrode or a laminate type obtained by laminating many electrodes is adopted.
For example, in the publication of Japanese Patent Application JP-A No. 59-108311 (Patent Reference 1), there is a description concerning the structure of a laminate type solid electrolytic capacitor electrode in which plural foil-shaped aluminum sheets formed into a specified shape and increased in surface area by etching are laminated and pressed to each other to constitute an aluminum foil laminate, and plural holes penetrating from the top surface to the bottom surface of aluminum foil laminate are formed by drilling.
In the cylinder type solid electrolytic capacitor, an aluminum plate or aluminum foil, or aluminum etched plate or aluminum etched foil provided with a dielectric layer as the anode is used and an aluminum foil or an aluminum etched foil is used as the cathode conductor. In the laminate type solid electrolytic capacitor, an aluminum plate or aluminum foil, or aluminum etched plate or aluminum etched foil provided with a dielectric layer as the anode is used and a graphite paste layer or a silver paste layer is used as the cathode conductor.
The electrolytic layer substantially acts as the cathode. As the material of this electrolytic layer, a functional polymer having high conductivity is used to actually develop a solid electrolytic capacitor having a low ESR. For example, a conductive polymer compound selected from the group consisting of polypyrrol, polyaniline, polythiophene, polyfuran and derivatives of these compounds and especially, polypyrrol, polyaniline, polyethylenedioxythiophene or the like is used for the purpose of developing an electrolytic layer having a low ESR.
However, aluminum material or aluminum etched foil to be used as the cathode conductor in the cylinder type solid electrolytic capacitor has the problem that the resistance (surface resistance) of the interface between this foil and the electrolytic layer is increased because a firm oxide film is formed on the surface thereof. This increase in surface resistance is a hindrance to the development of a cylinder type solid electrolytic capacitor having a low ESR.
In the meantime, it is necessary to enlarge the surface area per unit project areas of the anode and cathode conductors in the solid electrolytic capacitor to attain a small-sized and high-capacity capacitor.
For example, there is a proposal of a solid electrolytic capacitor electrode member in the publication of JP-A No. 2002-367867 (Patent Reference 2), the electrode member comprising an anodic body made of a valve action metal foil constituted of any one of tantalum, niobium and aluminum having a purity of 99% or more and an electrode layer made of a valve action metal powder formed on the anodic body, to attain a small-sized and high-capacity capacitor.
There is also a proposal of a solid electrolytic capacitor electrode member in the publication of JP-A No. 2003-272958 (Patent Reference 3), the electrode member comprising a low-melting point valve action metal foil constituted of a niobium foil and an electrode layer formed on the valve action metal foil by using a valve action metal containing tantalum having a higher melting point than the valve metal foil or its alloy powder, to attain a small-sized and high-capacity capacitor.
However, in these electrode members, the adhesion between the anodic body and the electrode layer is unsatisfactory, so that there is a limitation to these electrode members in increasing the surface area per unit project area, and these electrode members therefore cannot comply with the request for a small-sized and high-capacity capacitor.
In the meantime, the electrode used in an electric double layer capacitor is formed by coating a current collector with an electrode layer prepared by kneading activated carbon and a carbon powder together with a binder. As the current collector, a metal plate or a metal foil of aluminum, copper, stainless steel or the like is used in general.
In order to obtain an electric double layer capacitor having a large capacitance, it is necessary to increase the contact area between the polarizing electrode and the electrolyte by forming a thick electrode layer on the surface of the current collector. However, if the thickness of the electrode layer is increased, there is the problem that the ratio of the capacitance to the active material that is a capacitance per unit weight of the active material existing in the electrode layer is decreased.
Also, if the thickness of the electrode layer is increased, there is the problem that the adhesion in the inside of the electrode layer or at the interface between the electrode layer and the current collector is reduced.
Moreover, if the thickness of the electrode layer is increased, there is the problem that the surface resistance of the electrode is increased and therefore, the ESR (equivalent series resistance) of the electric double layer capacitor is increased. When the ESR of the electric double layer capacitor is increased, it is difficult to constitute a high-output electric double layer capacitor used for a power source for hybrid electric cars.
There have been many studies as to a method of raising the capacitance-to-active material ratio. However, it is necessary to use a binder to finally fix the active material in the current technologies and there is therefore a limitation to an increase in the capacitance-to-active material ratio. Also, if the amount of the binder is decreased to raise the capacitance-to-active material ratio, cracks and breakdowns of the electrode layer are easily caused when manufacturing, particularly, electrodes such as cylinder type electric double layer capacitors needing a high capacitance. It is therefore difficult to increase the capacitance-to-active material ratio of the electric double layer capacitor.
Various studies have been made as to a method of improving the adhesion between the electrode layer and the current collector.
In the publication of JP-A No. 10-223487 (Patent Reference 4), there is a disclosure of an electric double layer capacitor electrode produced by applying a polarizing electrode member to one surface of a metal foil whose surface is treated by etching to form a polarizing electrode layer and then by reducing the thickness to 80% to 90% the original thickness by rolling.
The publication of JP-A No. 2000-348987 (Patent Reference 5) discloses a method of manufacturing an electric double layer capacitor electrode by overlapping a flexible metal foil and an activated carbon cloth prepared by making fibrous activated carbon into a fabric form on each other and pressing the both against each other to produce a double layer electrode member provided with the metal foil layer and the activated carbon layer having a given thickness, wherein after the metal foil is overlapped on the activated carbon cloth, a press plate formed with plural projections is brought into contact with and pressed against the metal foil side to press-fit the projections to the inside of the activated cloth through the metal foil.
Disclosed in the publication of JP-A No. 2002-175950 (Patent Reference 6) is a method of manufacturing an electric double layer capacitor electrode body in which an electrode layer containing a carbonaceous material and a first binder is formed on at least one surface of a metal current collector foil, the method involving the following steps A to D. Here, the step A is a process of applying a conductive adhesive containing a conductive powder, a second binder and a solvent to at least one surface of the metal current collector foil, the step B is a process of drying the metal current collector foil coated with the conductive adhesive to remove at least a part of the solvent, thereby forming a conductive adhesive layer, the step C is a process of manufacturing a sheet-like molded body containing the carbonaceous material and the first binder, and the step D is a process of placing the sheet-like molded body on the conductive adhesive layer to form a laminate of the metal current collector body and the sheet-like molded body and by rolling the laminate to reduce the thickness of the sheet-like molded body by 5 to 60%, thereby forming an electrode layer comprising the sheet-like molded body.
Also, there is a disclosure as to a method of manufacturing an electric double layer capacitor electrode sheet in the publication of JP-A No. 2004-186194 (Patent Reference 7), the method comprising carrying out a step of manufacturing a lengthy sheet-like electrode having a given thickness from a molding material containing a carbonaceous powder, a conductive adjuvant and a binder and then a step of laminating the sheet-like electrode on the surface of the lengthy conductive foil through a conductive adhesive, wherein the laminating step is carried out by laminating the sheet-like electrode with applying the conductive adhesive to the surface of the conductive foil in a thickness of 10 μm or less by using a gravure coater.
However, in the electric double layer capacitor electrode disclosed in each publication of JP-A Nos. 10-223487 and 2000-348987 (Patent References 4 and 5), it is difficult to integrate the current collector with the electrode layer because oxides and hydroxides exist at the interface between the metal foil as the current corrector and the electrode layer when the both are stuck to each other.
Also, in the electric double layer capacitor electrode body or electric double layer capacitor electrode sheet disclosed in each publication of JP-A Nos. 2002-175950 and 2004-186194 (Patent References 6 and 7), it is difficult to integrate the current collector with the electrode layer because the existence of a conductive adhesive as the adhesive agent is inevitable.
For this, there is the case where such a phenomenon occurs that the electrode layer is peeled from the current collector when the electric double layer capacitor is charged or discharged. This resultantly gives rise to the problem that the charge and discharge characteristics of the electric double layer capacitor are deteriorated.
Patent Reference 1: Publication of JP-A No. 59-108311
Patent Reference 2: Publication of JP-A No. 2002-367867
Patent Reference 3: Publication of JP-A No. 2003-272958
Patent Reference 4: Publication of JP-A No. 10-223487
Patent Reference 5: Publication of JP-A No. 2000-348987
Patent Reference 6: Publication of JP-A No. 2002-175950
Patent Reference 7: Publication of JP-A No. 2004-186194